The broad objective is to study the physiology of the mammalian kidney, in particular that of the renal medulla. Our investigations center upon three general topics: urinary concentrating mechanism, microcirculation of the renal medulla and functional nephron heterogeneity. Micropuncture and related microanalyses and television microscopy are used to study normal rodents and and two special strains of rats--hereditary diabetes insipidus (Brattleboro) and long renal papilla (Munich Wistar). Urinary concentrating mechanism: 1) Determination of chloride concentration gradient across the thin loop of Henle. 2) Determination of Na, Cl and urea gradient across the thin loop of Henle in diuresis compared to ADH-induced antidiuresis in Brattleboro rats. 3) The effect of intravenous protein infusion on urinary concentrating ability. 4) Intracortical location of the glomeruli supplying the long loops of Henle in the papilla. Microcirculation of the renal medulla: 1) Determination of the red cell velocity in vasa recta capillaries in several conditions--normal hydropenia, water diuresis, and ADH-induced antidiuresis. Nephron heterogeneity--potassium secretion by the juxtamedullary nephron: We plan to determine the fraction of filtered potassium remaining at the end of the descending limb in potassium loaded rats to determine if medullary potassium re-cycling between the thin ascending and descending limb is involved in potassium adaptation. BIBLIOGRAPHIC REFERENCES: Jamison, Rex L., Veeraf M. Sanjana, J. Phillip Pennell, and Frank B. Lacy. Potassium sectetion by the descending limb or pars recta of the juxtamedullary nephron in vivo. Kidn. Intern. 9: 323-332, 1976. Jamison, Rex L., Paul A. Johnston, Frank B. Lacy, and Channing R. Robertson. The source of inulin in plasma from vasa recta capillaries of the renal medulla in vivo. Clinical Research 24: 162A, 1976 (Abstract).